(Fminibuffer_complete_and_exit): Supply value for new

(Fminibuffer_complete_and_exit): Supply value for new
ESCAPE_FROM_EDGE parameter to Ffield_beginning.
(Fminibuffer_complete_word): Use
Ffield_beginning to find the prompt end.
(Fminibuffer_complete_and_exit): Test for an empty
input string by seeing where the field begins, instead of
looking at text-properties.
(read_minibuf): Don't save minibuffer prompt length on
minibuf_save_list.
Don't initialize minibuffer prompt length.
Wrap prompt text-properties around the entire prompt.
Add 'prompt text-property to prompt.
Get final value with Ffield_string instead of make_buffer_string.
(read_minibuf_unwind): Don't restore minibuffer prompt length from
minibuf_save_list.
(do_completion): Get minibuffer input with Ffield_string
instead of Fbuffer_string.
Erase minibuffer input with Ferase_field instead of erase_buffer.
(Fminibuffer_complete_and_exit): Likewise.
Test whether buffer is empty by looking for the 'prompt text
property at the end.
Set prompt length by looking for the end of the prompt text property,
and save prompt length for later use (since there is no longer a
buffer variable to get it from).
(Fminibuffer_prompt_width, Fminibuffer_prompt_end): Functions removed.
(syms_of_minibuf): Remove initializations of
Sminibuffer_prompt_width and Sminibuffer_prompt_end.